What recombination is and why it matters

-A curious adult from Texas

February 5, 2018

Recombination is a big reason we are all unique. It is also a way to keep our DNA from building up too many mistakes. Recombination can repair broken DNA.

That is the why. To get to the how, we are going to need to talk a bit about our DNA and how it is organized.

DNA is Packaged in Chromosomes

Your DNA has the instructions for making you. If we stretched out those instructions, they’d be over 6 feet long.

Recombination goes something like this:


In this image, the chromosomes are shown as X’s. The darker blue-green one is one chromosome in the pair and the light blue is the other. (You can ignore the a’s, b’s, and c’s.)

Let’s go over how recombination resulted in the chromosome 1 result.

Let’s say these are mom and dad’s chromosome 1’s:

Dad has a blue and an orange one. I have drawn the two mom’s in gray because we won’t be following those. We will only look at the shared DNA from dad.

Now let’s see what happens when dad’s DNA is passed down to the half-brother and the half-sister:

reprogramming DNA in somatic cell nuclear transfer

-A high school student from Tennessee

January 29, 2018

The really short answer is that scientists don’t change the DNA itself. The egg has the stuff it needs to read the parts of the DNA instructions necessary to make an embryo.

The egg can “open up” or “unroll” the parts of the DNA needed to get the egg to start on the journey towards developing into a baby. To do this, it uses something in the egg called “pioneer factors.”

Different Cells Use Different Parts of the Same DNA

DNA is Rolled Up in Proteins

Obviously, our genes aren’t paper wrapped around a stick. However, the truth isn’t too far off. The DNA that makes up a gene is wrapped around individual proteins called histones.

Remember the individual instructions in the DNA called genes? Well, they don’t do anything as DNA.

Instead the instructions are read by the cell and ultimately turned into proteins. It is these proteins that do the work in a cell.

Histones are one of these proteins. They are the “spool” around which DNA is wound.

However, some transcription factors, the “pioneer factors” can actually open up the DNA.  Yes, scientists really call them pioneer factors.

Like the pioneers of the old west, they find where they need to go all on their own, settle down, and other factors come to them. Pioneer factors push the histones out of the way so the other proteins can read the DNA.

Eggs are Full of Pioneer Factors

Before eggs are fertilized, their DNA isn’t being read. Mom gave the egg all the proteins it would need to start going from one cell to a full organism.

Genetic tests imprecise for complex disease

-A curious adult from California

January 19, 2018

There are lots of reasons genetic tests can’t perfectly predict your risk for Type 2 diabetes right now.

One is that this is the sort of disease where the environment plays a big role. Even with great genetics, if you don’t exercise and eat Twinkies all day, you’ll be at a greater risk for Type 2 diabetes. And someone with terrible genetics who runs triathlons and is a vegan may be at a lower risk than you are with your poor habits and great genetics.

Coffee Jitters: A Simpler Case

The thing in coffee that gives it its oomph is caffeine. It can perk you up when you are run down.

But caffeine doesn’t last forever. Eventually it wears off and you need another cup. Or two.

Caffeine peters out because your body slowly breaks it down. And your genes determine how well your body does this.

This adds a layer of complication.

Imagine your body makes the super-powered gene that breaks caffeine down fast. Some genetic tests will tell you that you can handle a lot of caffeine. But you know this isn’t right because you get the jitters from just a single cup.

This is where AHR might play a role. Imagine that because of your AHR gene, you actually make very little of your super-powered caffeine destroyer. Now even though you have that powerful version of the first gene, your wimpy copy of the second gene sabotages the whole thing.

Half vs full sibling best test

-A curious adult from California

January 8, 2017

If you want to tell if someone is your full or half-sibling, the best test is probably from 23andMe.

Here are the results from a half-brother, a brother, and someone who is unrelated:

Full vs. Half, Yes. Half vs. Nephew, No.

So the 23andMe results are pretty easy to read.

If there are blocks of black in the picture, he is your full brother. No black or light blue means unrelated.

And if there are only light blue blocks, he may be your half-brother. Or he might be your uncle or nephew or grandfather or grandchild or maybe even first cousin!

Not Just Siblings

As you may have gathererd from the discussion so far 23andMe test isn’t just good for sibling vs. half-sibling either. It can tell if someone is your aunt, your second cousin or your double first cousin.

But as I have alluded to, it isn’t perfect.

It is excellent at telling whether two people are related or not. It can almost always tell up to second cousins and sometimes even fifth or ninth cousins.

Two genomes per cell

-A curious adult from Nevada

December 13, 2017

Your DNA has the instructions for making you. One complete set of instructions is called a genome.

These instructions are written in “letters” called base pairs. A human genome has around 3 billion base pairs.

The reason you sometimes see the 6 billion number has to do with how your DNA is stored in your body.

Your body is made up of trillions of cells. Each of these cells has two copies of your genome.

Sperm and Eggs Have One Genome

Are you still wondering why sperm and eggs only have one genome copy? Well, maybe it’s helpful to talk about the types of cells in the human body.

In general, there are two categories of cells. Most of the cells in our body are called somatic cells. Somatic cells have two genome copies.

Egg and sperm cells make a different category called germ cells. Germ cells start off with two genome copies. Eventually they divide into two cells with one genome copy each. This process is called meiosis.

Not All Cells Have Genomes!

So we know that the cells in our body have two genomes and germ cells have one genome. Every genome is housed in an area of the cell called the nucleus. But there’s more to it.

Not all of the cells in our body has copies of the genome. Red blood cells, and some cells in our skin, hair, and nails don’t have any genomes In their nuclei. These cells start off with genomes in order to develop properly, but then destroy their nuclei later on.

transgender intersex sex chromosomes

-A graduate student from Wisconsin

December 5, 2017

What a great question! Yes, there are some genetic conditions that can make someone not clearly fit into the category of “male” or “female”.

There are people who have differences in their sex chromosomes, or the X’s and Y’s you are talking about. There are also people who are intersex, with bodies that don’t fit typical definitions of male and female.

Sex Chromosomes and Genetics

Chromosomes are the instruction manuals of our body, and help determine things like our hair color, eye color, and body parts. Some chromosomes are called sex chromosomes which help to determine whether someone has the reproductive body parts of a boy or a girl.

Usually, someone who has female body parts has two X chromosomes, and someone who has male body parts has an X and a Y chromosome. Sometimes, this is not the case.

Being Intersex Is…

Now, there are other genetic conditions that have to do with people’s gender that don’t involve extra or missing X’s and Y’s. There are genetic conditions that cause people to be intersex.

Instead of having whole extra or missing chromosomes like with the sex chromosome conditions, many people who are intersex have specific changes in one of their genes.

Attached ear lobes genetically complicated

-An elementary school student from California

December 1, 2017

I can see why you’re asking.

We are taught in school that parents with attached ear lobes can’t have a child with unattached or hanging ones. We are taught wrong.

Parents with attached ear lobes can have kids with unattached ones. And vice versa.

Not the Bit of DNA You are Looking For

A really important point to make is that this study did not find the bits of DNA that actually affect your ear lobes. Instead they found the area of your DNA where the important bit might be hiding.

It is like in those spy movies where the CIA can narrow down where a phone call is coming from to a single city block. They know the bad guy is somewhere in that block but they don’t know on which floor of which building.

Why green peas are recessive and common

-A high school teacher from Pakistan

November 16, 2017

Because that is what we have chosen. Basically we decided we liked our peas green and so we only breed pea plants that have green peas.

There is no “yellow” gene in the peas found on farms. And unless we introduce one or one arises through mutation, there won’t be any. Green peas will be all there is forever.

Keeping Green Peas Common

The instructions for making a pea plant are found in its genes. Each gene has the instructions for one small part of the pea plant. There is a gene that determines the color of the peas the plant will produce.

Genes can come in different versions. The pea “color” gene comes in a yellow version (Y) and a green version (y).

Just like us, pea plants have two copies of most of their genes. This means that there are three possible combinations for this color gene:

Changing Ratio of Yellow and Green

Let’s start out with our early population:

Remember, we have 6 plants making yellow peas and 2 making green.

One way we can change things is what happened on farms all over the world. We can keep the plants with green peas and kill off the ones that make yellow. Now we have all green peas:

Best avuncular genetic test aunt uncle nephew niece

-A curious adult from Florida

November 9, 2017

The internet is certainly clogged with lots of genetic tests claiming they can help you tell if two people are aunt/nephew. Unfortunately, most can’t really give a definitive result every time. Or even most of the time.

Luckily there are much, much better tests out there. These tests don’t go by the official “avuncular” name because they can tell lots of different relationships in a single test. They are usually called ancestry or relatedness tests.

Few vs. Many

Most people’s DNA is split into 23 pairs of chromosomes. Each chromosome is one long piece of DNA.

A standard test looks at a spot here and a spot there amongst these 23. A good avuncular test will look at 30, 40, or even 50 different spots.

The test sees how many of these markers (as these spots are called) two people have in common and figures out relatedness that way.

This is perfectly fine for paternity cases. But for more distant relationships, even 50 isn’t enough.

Why Turner syndrome has symptoms

-A curious adult from Saudi Arabia

November 1, 2017

It is true that one X chromosome is inactivated in women. But the key is that it is not completely shut off. Some of that second X is still turned on.

This is why women with Turner syndrome, who have a single X chromosome, have symptoms. They are missing that small part of the inactivated X that escapes the inactivation.

15% of Genes Still Working

Your DNA has the instructions for making and running you. These instructions are stored in 23 long stretches of DNA called chromosomes. The X is one of these chromosomes.

Most chromosomes come in pairs. So you have two copies of chromosome 1, two copies of chromosome 2, and so on.

If you have two copies of the X then you are usually biologically female and if you have one X and one Y, you are usually biologically male.

When One Isn’t Enough

One way to think about yourself is a chocolate chip cookie. (Bear with me here.)

Each gene is like one small part of the recipe. Add two cups of flour. Add two eggs. And so on.

In the end, if you have right number of ingredients and add them at the right time, you end up with chocolate chip cookies. And if you have the right number of genes that get turned on and off in the right way, you end up as a person without symptoms.